Heterodyne receiving system



Sept. 8, 1936. w. s. BARDEN HETERODYNE RECEIVING SYSTEM 2 sheets-sheet 1 Filed Feb. l2, 1932 0 6,9/0 VOA 746i INVENTOR WILLIAM S. BABDEN BY ATTRNEY A 2 Sheets-Sheet 2 w. s. BARlZnr-.Nl HETERODYNE RECEIVIG SYSTEM Filed Feb. 12, 1952v Sept. 8, 1936.`

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Patented Sept. 8, 1936 PATENT cleric Y l 2,053,414 i' Y HE-TE-RoD-YNEL- naosivme SYSTEM VWilliam Stoddard' Bar Island, N. Y., assigner den, Stapleton, staten to Radio Corporation of America, a corporation of 'Delaware Application Februaryj12,

1932, sriaiNo; 592,461

16 claims. (o1. 25M-26)" v lvl-"' present inventionl 'generally-'relates# to N'elec'- tr-ical "systems for producingwaves of fhighlfe-A quency,Y *and-" more particularly pertains toV methods of, and meansfor;receivingl modulated? Asis Wellknownl theA heterodyne method'of` fre` ceptionV consists `iii-combining incominghighL fre-v queney-'currents with ai locally-generated current of a `frequencywhichl difersffromthat of therelo'-ceivedv -cu'rrentlby an amount whichvv may liewithinv thea audible; orfthe superlaudibl'e, frequency? range. Thesuperhetrodyne method ofl recep'- tion" utilizes alfbeatpf or intermediate, frequencyV which 'lieswithin the supereaudiblerange, and in 1 modern* broadcast-receivers *a* special' tube'is "employedfor producingthe local oscillationsiu Additionally; it hasbeeniproposed inlthe'rpast to utilize La: single tubeandfassociated circuits, that is4 an autodyn'ef methodf for producing Kthe f local 2()1'. cs'cillati'ons;'andy heterodyning the signal currents WithE the `locally"generated currents Yto create the beat frequency currents? However, suchcombinjed'localoscillatorefreduencychanger, or compositest detectoreoscillator; arrangements J-have Ain-tlf'n pastibeen economical expedients rather; than eXaniplespfV improved" technique:` Such compositeldetctor-"oscillator 'arrangements have not, as far as operating performance'isI concerned, beeni comparable tosuperheterodyneA receivers, whicliutili'zed aospecial tube for Aprodl'icing the locaioscinations Careful` analysisVv of' composite detector-oscilL' lator circuits Awhich have hitherto been proposed reveals'tliefact thatf'one'of'the primary reasons "fofthe relatively poor operation-"ofthpis type'of circuit' resides in" a' particular type of distortion. It can be shownth'at When the' 'incoming' signal is applied' ftothe Acontr'o'lj gridcircuit 'of a composite detectoroscillator*ofithepriorfart, signals of in-Y itensity'sui'licient to swingjthecontrolgrid highlyv positive 'are not faithfully. reproduced'in the output'ci'rc'uitliof "the composite arrangement. That isv to say, and especially in" the case lof modern ibroadcastlreceivers:of the superheterodyne type, "such composite arrangements of the prior art are inherentlyincapable'ofdetecting those components of" al modulated carrier Wave whichcorre spend tothe highery audio frequencies.

Accordingly, it` isone of the principal objects of my 'presentfinvention tozprovidefa vreceiving systemj: operating? according;A toy the"v hetero'dyne method of recep'tion'gwherein modulated carrier waveslfare :detected-f-loy means ,Y of a' composite de-v 55gl'tector-Ioscillator ivv'hich is so designed :that f its output comprises fa faithful fre'p'roducti'on of `-the modulated carrier Wa'veimprssed upon its Sfinput.

Considering- "the abovem'entioned defect-1 inner# ent in compositedetectoroscillator circuitsfofthe past, it -canlbesliovvn that the electronidischarge 5`f tubes which have been employed in such compo's# itefcirci'lits*werev responsible for i theinabilifyV of the composite detector` fto:faithfullyl reproducethe modulated'carrier waveY received by it. Y I lia'v'e found that `th`efabs`ence off-control gridfburentis 510" an essentialifconditionLforia composite detectoroscillator of= 'great fldelit'yQwli'eni the f incoming signalis' applied tofthe control gridof th'e composite circuiti y Thusfit imaybe statedv that it is-"an equally imf15" portant object of the present invention to -p'r'od vide a radio receiver of the heterodyne-typ; and Whichre'ceive'r employs @composite detector-iosci1lator-- circuinf wherein'l A the composite circuiti utilizes an electron discharge-tubepossessinga' 20 dynamic characteristic 'suolir that theicntrolgrid of" the' tube isl incapable i of drawing-current throughout the impression of signalvl-tagesfupoi thesaid control gri'di'v Infolevising 'asuperheterdyn :receiver embody# 25' ing the composite first detecto-rioscillatorof? the* presentifinvention, it'l1as-'-been' fordithat the radio1 fequency' os'cillation'- circuit iitilizkjd infY conjunction With the first detector circuitfeds'- back energy fromtheanod'icircuit thereof to-its 30@ control grid circuit, and yet is easily adjusted'to oscillat'e Without causing control grid "currentto How. In other "vv/ord's; thepresentinvention? fur-v thermore"providsa-method fof, andv means for' producing oscillations? of radio frequency iin-'fa 135i circuit employing a-'tubedesigned'in'such afm'annerthat loading vofthe output 1circuit of the osi ciilat'or results iin a` dynamic' characteris't'i'cfwhose 'i uppery bend,or saturation-point, lies virithe megaltiv'e re'gonof' the control?'gridvoltagel'svvirigs;4l

The present' invention, additior'ially,`4 utilizes" "as a* practical embodiment off the aforementioned dynamic i characteristic," a circuit j employing an electronf discharge tube 'provided-with llive elec# tr'ode'sgascreen v'grid of posititepotentialfcommenl surateiwithf that of the anode potentialn beingierii ployed,`A` and a shield grid; aticathode potential,- be'ingldi'sposed between''tlie anode and v'screer'igrid;v Suoi-i a A velelectrodeitube, *al/sof'A referred tof` asl-afl radio frequencypentode' tube', possesses to a greatl 50 erdegreefthan'is found in" variousfotherv types 'oftubes, the latter usuallyi of, ther-triode and'screenVVV grilli-type',l a pronounced thirdorderr'effectin the characteristicfrelatingcontrol?gridyoltage to` al` ternating current load voltage;and-.this third-ora '553 each value ps.

characteristic lies .Withinn thevnegative region z of Obviously-1fl thefuseofl 'r amv extremelyhigh value-1 of .f platevpotentialv 2 maconsnection with a tube constructedfandl' desiguedftor. normally loperatev atA a platerpotenitial lvaluecfarl belowl the value representedin curvef-I ,.is; not -onlyfr` dangerous but results linzaufextremely:small'v spanv gridlpotential values.

oi liteforthe tube.

Considering.,thedynamic `characteristic-A O B5.: it-will be observed that theupp'er'saturation point 1 also lies in the positive region" of. gridffpotential variation. Clearly, as the externalxplate resiste` ance of the triode oscillationk circuitlis'decreasedV the upperisaturationy point ofthe curvel A 01B will be merelyfshifted upwards, Y butiIw-ill ;stilllie with;.

in the positive-regionof grid;potenti'alf.values;4 whileii the external yplate resistancefrwere in:- creasedf the saturation point.. would b'er-A shifted-vdownwards; but istill li'e withinrthex afor'esaidpos-vy itive region.

viewpoint, the` operating. characteristic-z ofial. tri-r ode employed in an oscillation circuit; wl'ienxnorfv` mally operating potentials arefused, possesses-annl upper saturation point lying -within the posltveve region `of grid vpotential values.

Iii-order to clearly represent' the eieetiofaff; dynamic characteristic, such -i as2Ay Or B; on a.=.`

composite first detector-oscillatonI circuit, .there is shown in the lower portionoFg. 1l three? groups of oscillations;v The-group designated'as-v Local oscillations represents ytwo cycles-of" loe cal oscillation voltage variation'.producedimthef output circuit of the composite detector=oscillaetor. Obviously, assuming a normal gridipotentiali Ec locatedat point'; on curve on curve-A O B32l the voltage swings of the-,locally generatediosel cillations should Avary--between'a minimum ,negaf tive value w, determined Sbyfthe lower saturation' point A; arida maximum positive value m0 dei ter-mined by the upper saturationpointzBf'.

Actual-ly, however, the maximzumtpositiveswlng of thelocally generated voltageis foundfto besome value an less positive-thanvthervaluefmof This reduction of peak-value, during the positive half of'the locally gen'eratedvoltageswing,v is:` caused by the ii'owof "grid current between the;y control grid and cathode of the-oscillating triode- .A In other words, it will' be seen:l thatitherlocally generated voltage peaks are cutoif during the.`

positive half 'of tlievoltage swings;Y

Consider, next,y the group of oscillations-'designated.4 asl Signal 2 oscillations??? cuit of the composite detector-oscillator,the-firstkv twov groups-of oscillations mentioned above being`` heterodyned in accordance with the-,well known, theory of heterodyne detection-r the'frequency/of`v the beat voltage variations being the difference between the local oscillation frequency and the signal oscillation frequency.

Without attempting lto yshown i anyquantitative,l

and ,exactl relation betweenthe three groups` 4tot oscillationsl shown. in 1 ,1 from al. qualitativeA Hence;V itl wi'lllbeobserved that; whether considered 1 from. :the Adynamic ,ora staticiNr Itv is.. assumed" that there is impressed upon the: controligridfof! In other` words, the f viewpointiltzwlllihefseenfthat; dueto :the,;1ed\1c+Vv ti'onr of'i'positi-ve pe'aknvaluefrof locally-.fgenerated:`

voltages 'iromxmato .uw lthere f-is f produced aacors Y respondingi reductionJ at pointsnalongi the -inter:; medi-'atesfrequency voltagenswings corresponding:

wpointsips otithefrsignallvoltage swings. Thatisn.

frequency f intensityrY points does r not.. aiiect the adiacenti. valueA off intermediate frequency. in; tensity1Sfh:r Thatfi's, thez'positive :voltage swing'l atpo'int Ss" ot the-signal oscillation doesrnot carry. thecontrolgrid offtheoscillating triode sum-.- cientl'y` positiveetc) draws' gridr current, and` hence thefvaluefSfii ista faithful. reproduction of the signal/voltage value- Ssr impressed; on .the control grid".1

However,4 it l will? be noted-S that the points of', maximumsignal 'intensity apparentlyf appear-to have the same intermediate. frequencyfintensity.l asf:ixnrnediately. adjacentI points of 'lower signal intensityE'Sgt Itiwill, therefore, be seen thatthe intermediate-vfrequency output 1, ofr the compositev` detectoroscillator x willil nothfaithfully reproduce the wavefform :impressed .upon its-input, as :re-

gards points of" maximum` signaliI intensity ps dhasta-dynamic characteristicv,` the tube operating; i

atf'normal potentials in the sense that conventional .values are-used, as `shown infFig. 1a; Thev tube whose -`dynamic characteristic is represented in- Fig.; 1w is`v a radio frequency" pentodev tube (such as the A-418), and 'possesses tof agreater degree*thanisfoundin various other v types of tubes apronounced third order'eiect; This third orderV effect existsby virtue oi normal pentode: properties whiclr may'v be` yemployed safely, rather thanfzbyvirtue of uncertainiproperties of asimilarfnature. wh'kih'4 aref found in connectionv with other-types oftubesi. Fon-example, thei normal operating-potentials ofithe:A type o tube: repre-L sented iri Fig: lainCludesA 901volts' for the. platepotential; volts for thezscreen grid potential and a -5 volts; for Athe control lgrid lbias;

It".y will 1 be observed'. that theziuppersaturationpoi-ntoL the characteristic of this pentode tube liesfwithin"thepnegative region 'ofI thel grid .voltage values, whereaseinFig; 11it1was shownthat itv wasnot until vexcessively dangerous `platepotentials-were employed, that theA upper. saturation point'was moved. into they negative region`4 of thegrid potential variations. Obviously, when a tube havinge: a.-Vw dynamic characteristic; ofthe type This detector-oscillator, the effect of grid current is absent, since the control grid can only be swung positive after the upper saturation point has beenreached. The result is a simple radio frequency oscillator which employs feed-back from the plate circuit to the control grid circuit, and which is easily adjusted to oscillate without drawing. grid current. The absence of grid current becomes the essential characteristic of the composite rst detector-oscillator of thepresent invention when the incoming signal is applied to the control grid circuit.

In Fig. 2 there is shown a circuit arrangement wherein the source of signal energy is conventionally represented, the source being coupled, as at M, to the resonant input circuit of a pentode tube constructed to have the dynamic characteristic shown in Fig. la., the tube being rep'- resented by the numeral 1. The input circuit 6 includes a shunt tuning condenser E', the control grid of the tube being connected to the high potential side of the tuning condenser through a coil L1. The anode of the pentode tube is connected to an energizing source supplying potential of 90 volts thereto, the anode being oonnected to its potential source through an antiresonant network 8, 8 in series with a second anti-resonant network 9, 9.

The network 8, 8 is maintained iixedly tuned to the desired intermediate frequency, the coil 8l being coupled to a similar network 8, resonant to the intermediate frequency. It is to be understood that the network 8 is connected to the input of a succeeding intermediate frequency amplifier. The amplifier may be followed by the usual remaining components of `a superheterodyne receiver such as a second detector, an audio frequency amplifier, and some type of reproducer, these various components not being shown, since they are well known to those skilled in the art.

The resonant net-work 9, 9 is capable of being variably tuned by the tuning condenser 9 to the local oscillation frequency, the frequency being of such a value at each setting that the difference between it and the signal frequency is equal to the desired intermediate frequency. For this purpose the condensers 9 and 6 are arranged for uni-control, as shown by the dotted line, and

it is to be clearly understood that any arrangef ment and device well known to those skilled in the art may be employed for maintaining the frequency difference between the network 6 and network 9, S constantly equal to the desired intermediate frequency.

It is not necessary for the `purpose of the present invention to go into such arrangements in detail, reference being made however to U. S. Patent No. 1,740,331 to W. L. Carlson. rIhe coil 9 of the local oscillator circuit is magneticallycoupled to the coil Li, thus providing a means for permitting local oscillation potentials to be impressed upon the control grid of the pentode tube 1. The fixed condenser l is connected in series between the low potential side o-f the condenser 9 and the low potential terminal of the coil 9, and functions as a by-pass condenser, and preferably has a value of 0.1 microfarad. It is to be noted that a point intermediate the condenser Il) and the variable condenser 9 is grounded.

Bias for the control grid of the tube is secured by means of a resistor II, shunted by a condenser IZ, vdisposed in the grounded leg of the cathode of the tube, such a grid biasing arrangement being well known tov those skilled in the art.

- shown in Fig. la is employed in a composite first The screen gridelectrode I3 is connected to a sourceof positive potential, and has preferably applied to it a potential of 90 volts, and may be connected to the same source to which the anode of the tube is connected. The shield grid I4 is disposed between the anode and screen grid I3, the shield grid I4 being preferably connected within the tube to a point on the cathode which represents the average potential of the cathode. Further constructional details of this type of tube need not be given at this point. The manner in which a pentode tube is constructed is well known to those skilled in the art.

In Fig. 3 there is Shown a modification of the arrangement shown in Fig. 2, the modification essentiallyconsisting in coupling the coil 9 of the local oscillation circuit to the coil L', the latter being arranged in this case in the grounded leg of the cathodecircuit instead of being disposed, as shown in Fig. 2, in series with the control grid. Otherwise, the circuit is exactly the same as shown in Fig. 2. It will be observed that in both the arrangements shown in Figs. 2 and 3, the coupling coil L is in the control grid circuit. It may be also stated at this point that the source of signal energy coupled to the network 6, 6 may be an antenna circuit, or even one, or more, stages of radio frequency amplification.

In Fig. 4. there is shown still another modification of the arrangements shown in Figs. 2 and 3. The arrangement in Fig. 4 is identical with those shown in Figs. 2 and 3 with the exception that the special coupling coil L is omitted, and, instead, the coil 9 of the local oscillation circuit is coupled magnetically, as at M, to the secondary coil in the tunable signal circuit 6. Clearly, any other arrangement may be employed for feeding back energy from the output circuit of the pentode tube 'l to its input circuit in order to impress locally generated voltage upon the control grid of the pentode tube.

The reactions and interactions taking place in the composite first detector-oscillator employing the pentode tube 1, and determining the dynamic characteristic shown in Fig. la, are so involved as to make desirable the use of mathematical formulae in elucidating the laws governing the phenomena taking place in the structure, and in particular in laying down rules of design whereby any one skilled in the art may construct the composite detector-oscillator of this invention.

Hence, consider the arrangements shown in Fig. 2 from the point of view of a simple amplifier wherein a signal at is applied across the coil L1 in the control grid circuit of the pentode tube i, and in whose plate circuit there is disposed a tuned impedance (wL)2 R is equal to substantially 50,000 ohms or more, the

interceptsof the corresponding load line drawn on a family of Ip v. Ep characteristics for various oftenfemployed inconjunction with grid current phenomena It is also to be notedthat the pen- ;negatvegridpotentials show a characteristic rerlatingplate .current to control ,grid voltage as shown in .'Fig. .1a.

"Employing :B 'volts .control vgrid bias,.as shown in 'Fig.1a, with afgridswing of A peak volts, .the

:plate current, and .hence the signal .potential -acrossthe load vat 271' Thissubtractive -quantity increases'as the cube ofthe applied signalmagnitude A, and, therefore, the potential across L (across the load resistance) may be considered as increasing with applied Asignal magnitude in anon-linear manner.

VDesignating no as the amplification from grid "circuit'to load impedancethen ,uo is seen to de- 'pend'uponjthe grid swing A, being the ordinate 'dividedby theabscissa at any point P (Considering potential across L plotted as ordinatesagainst signal magnitude A as abscissae) corresponding uto .whatever -particular value of A is involved, Obviously, if ,no wereplottedas ordinates against ,A as abscissae, then no would decrease ina linear fashion with increasing values of A, andP would represent any point along this linear relation.

Now, remove the external source of signal across.L1, and obtain the signal potential across 'L1 by coupling L1 to coilf9, resulting in ,M1, the

phase being such that self-oscillation is possible.

`Let the oscillatorupotential inthe grid circuit (and of course across L1) be A and also'let a apply at 'A VrIhen, the amplification of A appearing across coilS is MOA, and the vcurrent through the coil'S is wL y neglecting they veryslight effect of the coil're- "sistance- R.

that:

"It then 'follows that thefoscillation strength in the grid circuit' of tube 1 willbenautomatically `limited to -avalue 4whereat yThis-value of A is fxedby designing L and M suitably. Thus, itis that there is secured in the arrangementshown in Fig. 2a simple radiofrequency pentode oscillator -of the plate to grid y 75 'feed-back' type which is self-limited without grid current, and without platecurrentsaturation as tode oscillator of the present inventionisunique in that the plate, screen and control ygrid lvoltages may wellbenormal in the sense that con- 1 .ventional values ,are used, resulting in normal .high translation gain from radio frequencyto intermediate frequency.

`While t-I haveindicated and described several systems for` carrying my invention into. effect, it

will be apparent to one skilled in the art that my invention is by no means limited tol the particular organizations shown anddescribed'but-that ymany modifications may -be made without departing from the scope of my invention as set "forth in ther appended claims.

What I claims is: 1. In-a compositedetector-oscillator circuit an electron `discharge tube provided with an input and an output circuit, means for resonatingthe Y input circuit toa desired signal frequency, means for resonating the output circuit to a desired local oscillation frequency, means for resonating a portion of the output circuit to a 'desired heterodyne frequency, the output and input circuits beingA coupled to `produce local oscillations, and said tube having a dynamic signal grid Voltage-plate current characteristic .such that its upper saturation point lies in the Anegative region of signal grid Voltage variation, said tube having its electrodes maintained-at their normal operating potentials.

`2.V In a composite'*detector-oscillator circuit an electron discharge tube provided with 1an input and output circuit, ymeans forv resonating the-,input circuit to a desiredsignal frequency, means for resonating the output circuit to a desired `local oscillationfrequency, means forresonating a. portion of the output circuit to -a desired heterodyne frequency, the output and input circuits being coupled to produce local oscillations, and said tube. having a dynamic signal grid voltage-plate currentcharacteristic such that its upper saturationvpoint lies in the negative region of signal grid voltage variation, said tube comprising a positive screen grid disposed between the control grid and anode,- and a shield grid, at cathode potential, disposed between the screen grid and anode.

3. In a composite detector-oscillator circuit an electron discharge tube provided with an input and output circuits, means for'resonating the input circuit to a desired signal frequency, means for resonating the output circuit to a desired local oscillation frequency, means forvresonating a portion of the output circuit to a desired heterodyne frequency, the output and input circuits being coupledy to produce local oscillations, andA said tube having a dynamic signal grid voltage-plate current characteristic such that its upper saturation point lies in the negative region of signal lgrid voltage variation, said tube comprising a positive screen grid disposed between the control I,grid and anode, and a.A shield grid, at cathode potential, disposed between thescreen grid and anode, the anode, screen grid andcontrol grid being maintained at their normal operating potentials.

4. An oscillation circuit comprising an electron discharge tube provided with an input and output circuits, means for coupling the output and input circuits to produce oscillations, said tube being so lconstructed and designed as to possess a dynamic control grid Voltage-plate current characteristic circuit is loaded, said tube comprising a pentode whose electrodes are maintained at their normal operating potentials.

5. `In a superheterodyne receiver, a composite rst detector-oscillator circuit including an electron discharge tube having a tuned input circuit and a loaded output circuit coupled to the input circuit, said tube including in addition to the usual cathode,control grid and anode at least one cold electrode so related to the other electrodes that the tube possesses a pronounced third order effect in its dynamic signal grid voltage-plate current characteristic at normal operating potentials whereby there is a complete absence of grid current when the peak value of incoming signal is applied to the input circuit of the tube.

6. In a superheterodyne receiver, a composite iirst detector-oscillator circuit including an electron discharge tube having a tuned input circuit and a loaded output circuit coupled to the input circuity said tube being constructed to have a pronounced third order effect in its dynamic signal grid voltage-plate current characteristic whereby there is a complete absence of grid current when the incoming signal is applied to the input circuit of the tube, said tube comprising a pentode whose electrodes are maintained at their normal operating potentials.

'7. An oscillator circuit comprising a pentode tube including in addition to a cathode, control grid and anode a screen grid, and a shield grid at cathode potential, said tube having an input circuit and a loaded anode output circuit, and means for feeding back energy from the output circuit to said input circuit.

8. In a superheterodyne receiver, a composite detector-oscillator circuit including a tube having a signal input network connected between the control grid and cathode, a network in the anode circuit of the tube resonant to a desired local oscillation frequency, means for regeneratively coupling the anode circuit and the signal input network, a network in the anode circuit resonant to the operating intermediate frequency, said tube including at least a positive cold electrode between the cathode and anode, and said tube being of the type having a dynamic control grid voltage-plate current characteristic such as to maintain the said grid at a negative potential with respect to the cathode throughout the swing of signal voltage applied to said input circuit whereby no grid current flows in the control grid circuit at the peaks of said swing,

9. In a superheterodyne receiver, a composite detector-oscillator circuit including a tube having a signal input network connected between the control grid and cathode, a network in the anode circuit of the tube resonant to a desired local oscillation frequency, means for regeneratively coupling the anode circuit and the signal input f network through said cathode, a network inthe anode circuit resonant to the operating inter mediate frequency, said tube including at least a positive cold electrode between the cathode and anode, and said tube being of the Ytype having a dynamic control grid voltageplate current characteristic such-as to maintain the said grid at a a signal input network connected between the' control grid and cathode, a networkin the anode circuit of the tube resonant to a desired local oscillation frequency, means for regenerat-ii/cly coupling the anode circuit and the signal network, a network in the anode circuit resonant? to the operating intermediate frequency, said tube including at least a positive cold electrode between the cathode and anode, means for simultaneously tuning said signal and local oscillation networks through respective signal and oscillation frequency ranges, and said tube being of the type having a dynamic control grid voltage-plate current characteristic such as to maintain the said grid at a negative potential with respect to the cathode throughout the swing of signal volt-F age applied to said input circuit whereby no grid` current flows in the control grid circuit at the` peaks of said swing.

. l1. A high frequency circuit comprising a tube provided with a cathode, a signal control grid, 'an anode, and at least one cold grid electrode disposed between the cathode and anode, a signal input network connected between ground and said control grid, a connection from said cathode to ground including means for constantly maintaining the control grid negatively biased with respect to the cathode, a local oscillation network coupled between said cathode connection and one of the electrodes other than the cathode and control grid, and a connection from the anode to a network resonant to an intermedate frequency.

12. In a combined oscillator-detector network, a tube provided with a cathode, a signal grid, an anode, and at least a positive cold electrode between the control grid and anode, a signal input circuit connected between the cathode and signal grid, a coupling between the said input circuit and one of the cold electrodes of said tube other than said signal grid for producing local oscillations in said input circuit, a beat circuit connected to the anode of said tube, means for maintaining the said signal grid at a negative direct current potential with respect to said cathode, and the signal grid voltage-anode current characteristic of said tube being such that said signal grid is maintained at said negative potential even at substantially the peaks of the signal and local oscillation voltage swings.

13. In a combined oscillator-detector network, a tube provided with a cathode, a signal grid, an anode, and at least a positive cold electrode between the control grid and anode, a shield grid at cathode potential between said last electrode and the anode, a signal input circuit connected between the cathode and signal grid, a coupling between the said input circuit and one of the cold electrodes of said tube other than said signal grid for producing local oscillations in said input circuit, a beat circuit connected to the anode of said tube, means for maintaining the said signal grid at a negative direct current potential with respect to said cathode, and the signal grid voltage-anode current characteristic of said tube being such that said signal grid is maintained at said negative potential even at substantially the peaks of the signalv and local oscillation voltage swings. 14. In a combined oscillator-detector network, a tube provided with a cathode, a signal grid, an anode, and at least a positive cold electrode between the control grid and anode, a signal input circuit connected between the cathode and signal grid, a coupling between the said input circuit;

and one of the cold electrodes of said tube other than said signal grid for producing local oscillations in said input circuit, a beat circuit tuned to a super-audible frequency connected to the anode of said tube, means for maintaining the said signal grid at a negative direct current potential with respect to said cathode, and the signal grid voltage-anode current characteristic of said tube being such that said signal grid is maintained at said negative potential even at substantially the peaks of the signal and local oscillation voltage swings.

15. In a combined oscillator-detector network, a tube provided with a cathode, a signal grid, an anode', and at least a positive cold electrode between the control grid and anode, a signal input circuit connected between the cathode and signal grid, a coupling between the said input circuit and one of the cold electrodes of said tube other than said signal grid for producing local oscillations in said input circuit, a beat circuit connected to the anode of said tube, means in the cathode circuit of said tube for maintaining the said signal grid at a negative direct current potential with respect to said cathode, and the signal grid voltage-anode current characteristic of said tube being such that said signal grid is maintained at said negative potential even at substantially the peaks of the signal and local oscillation voltage swings.

16. In a combined oscillator-detector network, a tube provided with a cathode, a signal grid, an anode, and at least a positive cold screen electrode between the control grid and anode, a signal input circuit connected between the cathode and signal grid, a coupling between the said input circuit and one of the cold electrodes of said tube other than said signal grid for producing local oscillations in said input circuit, a beat circuit connected to the anode of said tube, means for maintaining the said signal grid at a negative direct current potential with respect to said cathode, and the dynamic signal grid voltage-anode current characteristic of said tube being such that said signal grid is maintained at said negative potential even at substantially the peaks of the signal and local oscillation voltage swings.

WILLIAM STODDARD BARDEN. 

